Getting the direct objects contained in a collection

Question

I'm using collection.objects to ask for the objects in a collection, and it gives me the whole object hierarchy. How do I get just the root of the hierarchy? So for example, I have a collection named "Models" and I import a model asset which has a transform hierarchy:

In code I then want to go through my models and transform them say. But I obviously only want to transform the root objects (i.e. "Building"). If I ask for the objects in the "Models" collection I get all of the objects (building, Exterior, walls...).

Is there some way to ask for just the direct objects or to process all of the objects to get the root(s)? I can write the code to do this but it seems like there should be an easier way?

Answer 1

I went ahead and wrote the code to return the top-most objects from a collection.

def removeChildObjectFromList(objects):
    # This takes a list of Blender objects and removes any that have object(s) in the list above them in the hierarchy
    rootObjects = []
    voidParentObjects = set(objects)
    for objectNode in objects:
        isRootObject = True
        parentObject = objectNode.parent
        objectChain = set()
        while parentObject is not None:
            if parentObject in voidParentObjects:
                # Adding these to the voidParentObjects makes the method more efficient
                voidParentObjects.update(objectChain)
                isRootObject = False
                break
            objectChain.add(parentObject)
            parentObject = parentObject.parent
        if isRootObject:
            rootObjects.append(objectNode)
    return rootObjects

def getAssetRootObjectsFromCollection(assetCollection):
    return blenderUtils.removeChildObjectFromList(assetCollection.objects)

I still think it would be nice to have a way to ask a Collection for its direct objects; it obviously "knows" what they are as it's aware of the hierarchy when they're displayed in the View Layer.

Answer 2

As far as the collection is concerned, it isn't aware of the parent/child relationship between objects. You can however do something like this.

import bpy

col = bpy.data.collections["Model"]
for obj in col.objects:
    if obj.parent:
        continue
    do_something_on_parent(obj)

This might break if your topmost object in the collection has a parent relationship with an object that is outside of this collection, but it seems it isn't the case here.

See https://docs.blender.org/api/current/bpy.types.Object.html#bpy.types.Object.parent

And https://docs.blender.org/api/current/bpy.types.Collection.html#bpy.types.Collection